The optimum timing of the air intake and exhaust valves of an engine differ according to the running conditions of the engine. At high speed, for example, a large valve lift and a long valve opening period are required in order to obtain high torque, while at low speed, a comparatively small valve lift and short opening period are required. Due to these differences, the running conditions of engines such as car engines vary widely. It was therefore impossible to design a valve drive cam which gives optimum performance for all running conditions.
In Tokkai Sho 63-167016 (Koho) published by the Japanese Patent Office, a variable cam engine is proposed wherein several cams with different shapes are provided, and the optimum valve timing is obtained by selecting these cams depending on the engine running conditions.
In such a variable cam engine, high torque output is obtained from low to high speed by providing a low speed power cam which gives high torque at low speed and a high speed power cam which gives high torque at high speed, and changing over between the two depending on the engine speed. In addition, an economy cam which has a still smaller valve lift and shorter opening period than the low speed power cam has also been proposed to improve fuel cost performance of engines on partial load.
Conventionally, the change-over between these cams was made by simultaneously driving cam selection actuators provided for each cylinder of the engine.
A certain time was, however, required from the time when a cam change-over command was issued until the time when the change-over was actually completed, and as there were some cases when a change-over could not be made depending on the range of crankshaft angular positions, cam change-over was not necessarily completed at the same time for all the cylinders.
It was therefore difficult to specify precisely which cam was being used for each cylinder immediately after issuing a change-over command.
When a cam change-over is made, the air charging efficiency of a cylinder varies according to the cam characteristics even if the throttle opening does not change, so the required fuel quantity and ignition period also vary.
If therefore, it is not possible to specify precisely which cam is being used as described heretofore, it is also impossible to control the fuel supply quantity and ignition period to be optimum values. This lack of optimum control produces a torque shock after making a change-over and adversely affects the composition of the exhaust gas.